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تسجيل مستخدم جديدExamining the Location of the Magnetopause in an Undergraduate Lab
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Integrating areas of current research into undergraduate physics labs can be a difficult task. The location of the magnetopause is one problem that can be examined with no prior exposure to space physics. The magnetopause location can be viewed as a pressure balance between the dynamic pressure of the solar wind and the magnetic pressure of the magnetosphere. In this lab sophomore and junior students examine the magnetopause location using simulation results from BATS-R-US global MHD code run at NASAs Community Coordinated Modeling Center. Students also analyze data from several spacecraft to find magnetopause crossings. The students get reasonable agreement between their results and model predictions from this lab as well as exposure to the tools and techniques of space physics.
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The terrestrial magnetopause is the boundary that shields the Earths magnetosphere on one side from the shocked solar wind and its embedded interplanetary magnetic field on the other side. In this paper, we show observations from two of the Time Hist
ory of Events and Macroscales Interactions during Substorms (THEMIS) satellites, comparing dayside magnetopause crossings with flank crossings near the terminator. Macroscopic properties such as current sheet thickness, motion, and current density are examined for a large number of magnetopause crossings. The results show that the flank magnetopause is typically thicker than the dayside magnetopause and has a lower current density. Consistent with earlier results from Cluster observations, we also find a persistent dawn-dusk asymmetry with a thicker and more dynamic magnetopause at dawn than at dusk.
While pressure balance can predict how far the magnetopause will move in response to an upstream pressure change, it cannot determine how fast the transient reponse will be. Using Time History of Events and Macroscale Interactions during Substorms (T
HEMIS), we present multipoint observations revealing, for the first time, strong (thermal + magnetic) pressure gradients in the magnetosheath due to a foreshock transient, most likely a Hot Flow Anomaly (HFA), which decreased the total pressure upstream of the bow shock. By converting the spacecraft time series into a spatial picture, we quantitatively show that these pressure gradients caused the observed acceleration of the plasma, resulting in fast sunward magnetosheath flows ahead of a localised outward distortion of the magnetopause. The acceleratation of the magnetosheath plasma was fast enough to keep the peak of the magnetopause bulge at approximately the equilibrium position i.e. in pressure balance. Therefore, we show that pressure gradients in the magnetosheath due to transient changes in the total pressure upstream can directly drive anomalous flows and in turn are important in transmitting information from the bow shock to the magnetopause.
We consider the one-dimensional equilibrium problem of a shear-flow boundary layer within an extended Hall-MHD (eHMHD) model of plasma that retains first-order finite Larmor radius (FLR) corrections to the ion dynamics. We provide a generalized versi
on of the analytic expressions for the equilibrium configuration given in Cerri et al. (2013) [Cerri et al., Phys. Plasmas 20, 112112 (2013)], highlighting their intrinsic asymmetry due to the relative orientation of the magnetic field $mathbf{b}=mathbf{B}/|mathbf{B}|$ and the fluid vorticity $mathbf{omega}=mathbf{ abla}timesmathbf{u}$ ($mathbf{omega b}$ asymmetry). Finally, we show that FLR effects can modify the Chapman--Ferraro current layer at the flank magnetopause in a way that is consistent with the observed structure reported by Haaland et al. (2014) [Haaland et al., J. Geophys. Res. Space Phys. 119, 9019-9037 (2014)]. In particular, we are able to qualitatively reproduce the following key features: (i) the dusk-dawn asymmetry of the current layer, (ii) a double-peak feature in the current profiles, and (iii) adjacent current sheets having thicknesses of several ion Larmor radii and with different current directions.
We analyze the development and influence of turbulence in three-dimensional particle-in-cell simulations of guide-field magnetic reconnection at the magnetopause with parameters based on observations of an electron diffusion region by the Magnetosphe
ric Multiscale (MMS) mission. Along the separatrices the turbulence is a variant of the lower hybrid drift instability (LHDI) that produces electric field fluctuations with amplitudes much greater than the reconnection electric field. The turbulence controls the scale length of the density and current profiles while enabling significant transport across the magnetopause despite the electrons remaining frozen-in to the magnetic field. Near the X-line the electrons are not frozen-in and the turbulence, which differs from the LHDI, makes a significant net contribution to the generalized Ohms law through an anomalous viscosity. The characteristics of the turbulence and associated particle transport are consistent with fluctuation amplitudes in the MMS observations. However, for this event the simulations suggest that the MMS spacecraft were not close enough to the core of the electron diffusion region to identify the region where anomalous viscosity is important.
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